Old PCB Capacitors

Jesse69

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It seems like most old pcb boards use electrolytic capacitors. Don't electrolytic capacitors "dry out" and lose their spec over time? I read a library book and it said to quick check a capacitor use the ohmmeter and if the resistance is low you got a shorted or leaky capacitor. If the resistance is high it's probably ok. Then you check capacitance with the + probe to the capacitor's + lead. Does it matter to the reading if the capacitor is still soldered in place, - or can you still get a good capacitance reading?

I'm wondering because I'm measuring capacitors off my Stargate Board and they are all off. I don't think my $15 on sale Sears Craftsman DMM is bad... Like several 100 mfd 25vdc capacitors are reading .032 to .040 nF so they're way off or am I doing something wrong?

22mfd 16vdc - Main Board =>.055 nF reading

So do you guys usually replace old capacitors on old pcbs when you refurbish them? What effect do out of spec capacitors have?
 
If it aint broke, dont fix it. Caps are usually replaced on Pac Man boards to fix various issues including audio, among other things. But if the board works, leave it be.
 
My bad sound board sometimes misses sounds or gives bad sounds, and the caps are off. But I guess the sound ROM is bad.

When my main Stargate PCB was working it would reset. The three 100mfd 25vdc capacitors give a .032 to .040 nF reading.
 
You really shouldn't try to read capacitors while they are still soldered or mounted to the board. You will get the total circuit resistance and capacitance.

Electrolytic capacitors are still put on new PCB's, it depends on the what the electronic device it supposed to do. Large caps still have their place in circuits, but they are making surface mount electrolytics as well.

Luke
 
You can measure caps in circuit with an ESR meter.
You will have about 1 percent success finding bad caps with a ohm meter.
 
The caps on the input power are just there as filter capacitors. They help reduce any induced AC hum on the unshielded wires. It would be very unlikely that they were causing the reboots.

There are a number of much more likely culprits. Low voltage on the input power. Williams CPU boards require 3 regulated voltages for the 4116 RAMs. If any of those voltages begin to sag or show AC on them, the RAM will freak out and you will reboot.

Bad ribbon connectors from the ROM board to the CPU board will cause addressing issues that can lock the CPU until the watchdog resets the CPU.

Problems with the watchdog circuit can cause it to randomly reboot.

A bad transistor in the reset circuit can misfire and reset the CPU.

And on and on.

ken
 
Caps, a philiosophy

If I by a board and it looks like it was in a harsh environment with heat, smoke etc. I will inspect the board and recap it. I can find bags of caps really cheap at my local surplus place or on Ebay. ($2-$) Radio Shack will charge $2-$3 JUST FOR 1 OR TWO CAPS IN A BAG! It only takes perhaps 30 minutes to an hour to cap most boards and then they will probably work forever in a home environment.
 
I've always been in the "re-cap" camp, but lately I'm loosing my "faith"....

F.I. the Berzerk machine I recently got. I measured the AC voltage across the filter caps on the power supply with my calibrated Fluke DVM and I got about 0.01 Volts.
That is with the original caps of course.

So, it _looks_ like they work fine.

However, apparently what happens when the caps get older is that the ESR starts to rise.
 
I have never 'recapped' a game board. In most places, they're only used for filtering, especially when they're on every row of chips or on several chips in a row. They buffer out small transient spikes/dips in the supply voltage for the digital logic.

They serve frequency dependent functions in audio circuitry and are critical in the vector processing circuitry on vector games. These are places where change in value can have a noticeable effect.

Aside from just a few filter caps in monitors, EVERY cap is used for analog processing. The operating temperature of monitors is a lot higher than the game boards too, accelerating the 'drying out' process.

I'll find snapped off or broken caps on a game board and I'll usually just clip them off. I've seen a few game board caps fail by shorting out. Sometimes you can catch them smoking or pushing their guts out as they die.

A shorted cap on a Tron PCB the other day took out a fuse and cooked an inductor on the middle board. Replaced the fuse, clipped the cap, and rolled on.
 
I recapped two game pcbs in the last week. Both were megatouch bartop machines. These machine are cramped and generate alot of heat. One board had a ton of bad caps and the second had two or three. They have at least a dozen little electrolytics on them. Recapping the first board cured its resetting problem and the second board was done for safe measure.
I replace caps on pac boards all the time. I dont replace them all, just ones that are bad or marginal tested with the ESR meter. The caps in the sound section are often bad.
 
Most DMM's only read the smaller cap values less than 500 nF which is only .5mF so you are probably getting bogus readings on your 100Mf cap. Plus the fact that they are in circuit. Also most caps have a tolerance of around 20% Values are only part of the equation, you need to check ESR, and I like to check Dielectric absorb.
 
I'm like you. If it checks OK with my ESR meter it stays.


I've always been in the "re-cap" camp, but lately I'm loosing my "faith"....

F.I. the Berzerk machine I recently got. I measured the AC voltage across the filter caps on the power supply with my calibrated Fluke DVM and I got about 0.01 Volts.
That is with the original caps of course.

So, it _looks_ like they work fine.

However, apparently what happens when the caps get older is that the ESR starts to rise.
 
caps are cheap. i replace them, all of them whenever i find them.
be it game boards, monitor chassis or power supplies i change them. some might take exception to the cost and the necessity but i dont care. every new to me game gets new capacitors all around. i consider it to be a part of the restoration process.

then, at least my customers are getting a very nicely rebuilt game with many new parts. that, and a burn in period over a few days and i can be confident they are getting a very good quality product.
 
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